Description (Adapted and modified from applicant's abstract): Macrophage recognition of invading pathogens is essential for the initiation of the innate immune response and the subsequent development of adaptive immunity. Specific recognition of bacterial components is mediated by Toll-like receptors (TLRs) that are recruited to phagosomes where they initiate inflammatory responses. TLR4 recognizes lipopolysaccharide from Gram-negative bacteria, and TLR2 recognizes distinct components of the Gram-positive bacterial cell wall. In addition, TLR2 and 6 cooperate in the recognition of an array of other cell wall components from Gram-positive bacteria, Mycobacteria and yeast. In this proposal, the applicant seeks to define the mechanisms by which TLRs 2, 4 and 6 discriminate between pathogens, activate appropriate responses in macrophages, and orchestrate an effective immune response to infection in mice. He will structurally map the ligand recognition domains of TLRs 2, 4 and 6, and determine whether bacterial products activate these receptors by generating endogenous ligands. He will establish how TLRs are recruited to phagosomes, whether they are selectively recruited to inflammatory phagosomes, and whether their association with phagosomes have consequences for host defense. He will explore differences in the signaling pathways activated by TLRs 2, 4 and 6 in order to determine the biological consequences of specific pathogen recognition. The role of TLRs 2 and 6 in host defense will be further explored in mice that have macrophage/neutrophil-specific defects in these receptors.